The following are heretofore known.
(1) Traditional fermentation of tea leaves, and fermented teas including goishi tea, pu-erh tea, miang tea, etc.
(2) A fermented green tea obtained by the following process: bacteria of the species plantarum of the genus Lactobacillus (lactic acid bacteria), which can proliferate in tea leaves, are separated from tea leaves, and cultured in a liquid medium; and the tea leaves are immersed in hot water, allowed to cool, inoculated with the cultured bacteria and fermented for a predetermined period (Patent Document 1).
(3) A process that can produce a large amount of fermented beverage from tea or coffee, using a yeast and bacteria under easy production conditions and in a relatively short production time (Patent Document 2).
(4) Mucosal immunity is the first phylactic mechanism employed when a pathogen adheres to a mucous membrane (Nonpatent Document 1).
(5) Secretory IgA (S-IgA) in mucus has a defensive activity against pathogens such as bacteria, viruses, etc. (Nonpatent Document 2 and 3), and neutralizes toxins produced by microorganisms (Nonpatent Document 4).    [Patent Document 1] Japanese Patent 2876006    [Patent Document 2] Japanese Unexamined Patent Publication No. 1997-220054    [Nonpatent Document 1] Brandtzaeg, P. Curr. Top. Microbiol. Immunol. 146: 13, 1989    [Nonpatent Document 2] Czinn, S, J. et al., Vaccine 11:637, 1993    [Nonpatent Document 3] Renegar, K. et al., J. Immunol. 146: 1972, 1991    [Nonpatent Document 4] Brandtzaeg, P., APMIS 103: 1, 1995; Kilian, M. et al., Microbiol. Rev. 52: 296, 1988